In mammals, a large number of enzymes exist that metabolize drugs and other xenobiotics. Cytochrome P450s are among the most important of these enzymes that are involved in metabolism of most therapeutically-used drugs. In addition, P450s catalyze the metabolic-activation of chemical carcinogens. The P450s involved in xenobiotic metabolism are found in the CYP1, CYP2 and CYP3 families. Each of these families consist of two or more subfamilies. The fact that P450s can metabolically-activate toxins and procarcinogens in vitro implies that they are involved in toxicity and cancer. However, it is not known whether P450s are required for the toxicity and carcinogenicity of chemicals in an intact animal. The only experiments suggestive of a role for P450s in cancer etiology are indirect chemically-induced transformation assays in cell culture, and genetic experiments in mice involving the Ah locus. No direct evidence is available to establish that P450s are necessary for carcinogenesis in an intact animal model system. To assess the potential contribution of P450s to acute chemical toxicity and the process of chemical carcinogenesis, and to determine their roles, if any, in mammalian development and physiological homeostasis, P450-null mice were produced. Mice lacking expression of other carcinogen metabolizing enzymes like the microsomal epoxide hydrolase (mEH) and the quinone oxidoeductase (NQO1) were also made. Studies with CYP1B1-null mice have revealed that this P450 is required for the carcinogenic efforts of polycyclic aromatic hydrocarbons. Mice lacking mEH are also sensitive to the carcinogens thus indicating that the classic diol-epoxide route of carcinogen activation is the most important or carcinogenesis in vivo. In order to produce mouse models that can more accurately be used to predict human drug and carcinogen metabolism, P450-humanized mice are being prepared using bacterial artificial chromosomes. Transgenic mice carrying the human genes are bred with P450 null-mice to produce humanized mice. Mice expressing human CYP1A1, CYP1A2, CYP2E1, CYP2D6, CYP3A4, and CYP3A7 have been produced and characterized.